Automatic control mechanism for plasma welder

ABSTRACT

An automatic control device for plasma arc welding by use of a plasma torch fitted with a cathodic electrode and a gas blow nozzle, said torch being movable relative to a welding stock which is arranged as an anodic electrode, said device being characterized by the provision of a photoelectric sensor arranged to measure the inclination angle of the tail flame part of the plasma arc emerging from the backside of the welding stock while the welding operation is going on, the electric output signal from said sensor being fed to an electronic control circuit adapted for control of any one or more of the welding parameters such as the relative travel speed between the torch and the welding stock, the rate of plasma gas supply and the rate of the welding current.

{72] Inventors Hlsashi Hlshida; [56] Ref en e Cited Masakazu Maruyama,both of Hiratsuka, UNITED STATES PATENTS [2H A 1 No 32?: 2,994,7638/1961 Schultz 219/123 [22] 522 3 2 1970 3,102,946 9/1963 Fonberg219/123 [45] Patented Aug. 31, 1971 Primary Examiner-J. V. Truhe [73]Assignee Sumitomo Jukikai Kogyo Kabushiki Kaisha Assistant Examiner-J.G. Smith Tokyo, Japan Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak(32] Priority Feb. 3, 1969 [33] Japan {31] 44/7305 ABSTRACT: Anautomatlc control device for plasma arc welding by use of a plasma torchfitted with a cathodic electrode arid a gas blow nozzle, said torchbeing movable relative to a welding stock which is arranged as an anodicelectrode, said device being characterized by the provision of a [54] tag gg z MECHANEM FOR photoelectric sensor arranged to measure theinclination angle 3cm 10D of the tail flame part of the plasma areemerging from the "wing backside of the welding stock while the weldingoperation is [52] US. Cl 219/121 P, going on, the electric out ut signalfrom said sensor bein fed P 8 219/125 R to an electronic control circuitadapted for control of any one [51] Int. Cl 823k 9/06 or more of thewelding parameters such as the relative travel [50] Field ofSearch2l9/13l, -s eed between the torch and the welding stock, the rate of135, 121 P, 123, 125 plasma gas supply and the rate ofthe weldingcurrent.

' O. C. m,

POWER SOURCE ELECTRONIC COMPARATOR 34 1 I I r ROTOR c riiii i 37-" 3:.35 CONTROL cmcun 1 cmcun PLASMA GAS FLOW RATE CONTROL clRcun PATENIEUwas] nan SHEET 2 OF FIG. 9 I x m w 85 l'w IIHIHIIIH TORC R23-29 RESISTORc4-|2 CONDENSER AUTOMATIC CONTROL MECHANISM FOR PLASMA WELDER Thisinvention relates to improvements in and relating to an automaticcontrol device for plasma arc welding.

The plasma arcwelder is known per se which comprises a cathodicelectrode fitted in a torch, on the one hand, and an anodic electrodeconstituting the stock to be welded. on the other hand, a high voltagebeing impressed between the both electrode and a main or centralstreamof rare gas such as argon and a concentric protecting gas streamconsisting of an inert gas such as argon or the like being injected fromthe torch towards the welding stock. The main gas stream is heated up bythe electric energy, into a welding plasma are which is directed to thewelding spot, as is commonly known. In place of the rare gases, air maybe equally utilized as the working gas for the formation of the weldingplasma.

A first advantage of the plasma arc welding is such that the fusion ofthe welding stock is carried into effect at an amazingly rapid speedthanks to the high flame temperature of the plasma, thereby attaining asubstantially speeded-up welding. A second advantage of the plasma arcwelding resides in a possibility of the formation of a fine andconcentrated arc having a high energy density highly adapted forrealizing a highprecision welding, in a highly favorable waysubstantially without inviting various conventional drawbacks as met inthe old-fashioned electric welding, such as lateral shrinkage, angulardeformation or the like defects in the welded stock.

It is the main object of the invention to provide an automatic controldevice for the plasma arc welder of the above kind, capable of operatingin a highly sensitive way to the welding conditions occasionally beingcarried into effect, especially in the case of welding of heavy stocks.

It has been already ascertained that the physical conditions of thekeyhole which is bored through the welding stock by the plasma flameduring the welding procedure have a substantial influence upon theresults of the welding. The optimum formation of the keyhole dependsupon various parameters such as the relative travel speed of the torch,the flow rate of the plasma gas stream, the arc current, the thicknessof the welding stock.

During the welding, the plasma penetrates through the welding stock forthe formation of a keyhole, as was briefly referred to above, and a tailflame being observed as coming out of the outlet of the keyhole. It hasbeen already proposed to measure the illuminance of such tail flame as ameasure for the control of torch travel speed relative to the weldingstock, so as to attain the optimum welding conditions. However, theilluminance of the plasma depends upon various and numerous otherconditions which occasionally fluctuate and cannot be controlled, thusthe delivered signal corresponding .in any way to the controllablewelding conditions and being resulted in failure.

We have investigated into the plasma arc welding procedures and foundthat when the inclination angle of the plasma tail flame is relied upon,as a sole parameter for the control of the torch-traveling speed or thelike welding parameter or parameters.

As the control parameter, the relative travel speed of plasma torch, thefeed rate of plasma gas stream, the arc current or the like mayadvantageously. be adopted, although the invention is not limited onlythereto. I

Based upon the above observations, the invention in its broadestcoverage resides in such an automatic control device for plasma arcwelding, characterized by-the provision of a photosensing means providedfor unitary moving with a plasma torch relative to. a welding'stock,said photosensing means being arranged for measuring the inclinationangle of the tail flame part of the welding plasma and the electricsignal being fed to a welding parameter control means which iselectrically connected with said measuring means.

These and further objects, features and advantages of the invention willbecome more apparent when read the following detailed description of theinvention by reference to the accompanying drawings, in which:

FIG. 1 is a partially sectioned schematic view of a plasma arc welderfitted with the automatic control device according to the invention.

FIGS. 2-5 are several schematic views illustrative of the inclined angleof the tail flame part of welding plasma torch relative to a weldingstock.

FIG. 6 is a substantially longitudinal section of a photosensing meanscomprising a pair of photocells designed and arranged to measure theinclination angle of plasma tail flame.

FIG. 7 is a connection diagram of an electric control circuit employedin the automatic control device according to this invention.

FIG. 8 is two diagrams illustrating the function of the pulsegeneratingsection employed as a part of the circuit shown in FIG. 7 for control ofthe travel speed of a welding torch relative to a welding stock.

FIG. 9 is a schematic sectional view of a control valve assembly adaptedfor the control of the flow rate of plasma gas stream.

FIG. 10 is a connection diagram of a modified control circuit designedand arranged for control of the welding current in place of the torchtravel speed.

Referring now to FIGS. 1-5, the main principles of the invention will bedescribed in a rather simplified form.

In FIG. 1, the numeral 10 denotes generally a plasma torch unit oftheconventional design which is attached fixedly at the free end of a rigidsupporting arm 11 carried on a wheeled carriage 12, the latter beingmovable along guide rail means 13 shown only partially.

The carriage 12 is provided with traveling wheel means 119 mounted onsaid guide rail means 13, said wheel means being mechanically coupledthrough reduction gearing, not shown, with a DC drive motor 45. Thismechanical coupling is shown only in a simplified way by a singledottedline at 116.

The torch unit 10 comprises as conventionally a tungsten cathode 14which is electrically connected through a lead 18 with the negative sideof a DC high-voltage source 15.

The unit 10 is formed with a gas chamber 10a which is connected througha conduit 1012, only shown in a simplified manner, by a dotted line,with a supply piping 16 connected in turn to an argon gas reservoir, notshown, for supply of plasma arc gas, as will be more fully describedhereinafter. The are gas may be argon, krypton, xenon, or a mixture ofany of these rare gases, or alternatively air. An inert protecting gas,such as argon, is fed from a proper reservoir, not shown, through aseparate supply piping l7 and a duct 57 shown only by a dotted line, toan auxiliary orifice 20 which is formed in the bottom wall of said torchunit 10 and around a main orifice 49 also bored through the same bottomwall.

Below the movable torch 10, there is provided a stationary welding stock29 only schematically and partially shown, said stock being electricallyconnected through a lead 21- to the positive side of the power source15, thus the stock being arranged to act as an anodic electrode. Theinside diameter of the main orifice 49 is selected to 2 mm., as anexample, and the distance between the bottom surface of the unit 10 andthe welding stock 29 amounts, by way of example, to 7 mm. Within theunit 10, there is formed a cooling water chamber 22 which is connectedwith a water inlet tubing 23 and an outlet tubing 24 for the circulationof the coolant.

Below the stock 29 and apart several millimeters therefrom, there is aphotoelectric angle measuring unit, generally shown at 25,-wh ichcomprises a pair of photoelectric cells 250 and 2517, as most clearly beshownin FIG. 6. These cells are electrically connectedthrough leads26-29'to an electronic comparator-30 which'is shown in'FIG. 1 only by arectangular comparator'30 is connected through leads 32 and 33 to aconventional amplifier 31, shown only schematically in FIG. 1 by ablock, but shown'more specifically in FIG. 7. The output of thisamplifier 31 is electrically connected through lead means 34 to rotorcurrent control circuit 35 which is also shown more specifically in FIG.7, this circuit being arranged to control the rotational speed of themotor 45 and thus the speed of the carrier 12 and the torch unit 10which is travelingin this case from right to left in FIG. 1, as shown byan arrow 36.

As an optional measure, the output of the amplifier 31 is connectedthrough lead means 34, 56 with an arc current control circuit 37, orthrough lead means 34, 38 with plasma gas flow ratecontrol circuit 39,as will be more fully described hereinafter.

The welding stock 29 consists of two adjoining steel plates 29a and 29b,as more clearly be seen from FIGS. 2-5.

In FIG. 2, a standard plasma arc welding condition is schematicallyshown. The plasma arc 40 is shown as being delivered from the lower endof the torch unit 10 upon the welding stock 29 against a welding spotwhich movably formed along the confronting gap formed between theadjoining welding stock elements 29a and 29b, said are penetratingthrough the stock 29 so as to form a rearwardly inclined keyhole 41,when assuming the unit 10 advancing in the direction shown by an arrow36'. The numeral 42 represents the tail flame which forms a rearwardlyinclined angle With the carriage l2 and the torch unit traveling at atoo much slower speed than that specified for the optimum weldingconditions shown in FIG. 2, the inclined angle of the tail flame 42 willbe increased to a larger value 0 as shown in FIG. 3. In this case, ablown-off of the material of stock 29 may frequently encounteredwithout'attaining the desired welding effect. This angle 0, will besubstantially nearer to a right angle.

With a substantially larger traveling speed of the torch unit 10 carriedby the carriage 12, the inclination angle will become a smaller valuesuch as at 6 shown schematically in FIG.-4.

From the foregoing, it will be clear that the following relation will befound among these several angular values:

In this case, the time period attributed to the fusing the weldingmaterial will become shorter than the properly allotted, and in extremecases, no properly shaped keyhole will be produced;

In FIG. 5, the aforementioned several working modes and thecorresponding tail flame angles are shown in an overlapped manner. Inthis figure, the photoelectric cells 25a and 25b are so positioned thatonly the latter cell 25b senses the issuing light from the plasma tailflame having an inclination angle 6 which corresponds to the optimumoperating conditions, while the former element 25a is so positioned thatit senses the light issuing from the tail flame having an inclinationangle 0, satisfying the following mathematical relation:

0 l9 90 and corresponding to an unfavorable welding condition where therelative travel speed of the torch is too much higher than the specifiedvalue.

The relative position of the photoelements 25a and 25b is fixed andmovable in synchronism of travel of the torch unit 10. For this purpose,these cells 25a and 25b are rigidly supported through a certainsupporting member similar to that shown at 11 in FIG. I, although notspecifically shown, by the carriage 12, as hinted by a chain line 26.

Electric output from either of the photoelectric elements 25a and 25b isfed through amplifier 31 to control circuit 35. As will be more fullydescribed hereinafter with reference to FIG. 7, when the first element25a senses'the light from thetail flame, the control circuit 35 operatesin such away that the when the second element 25b senses the light fromthe tail flame, the necessary control is carried out in the oppositesense which means that the torch travel speed is subjected toretardation.

travel speed of torch unit 10 is increased. On the other hand,-

The photocell-type angle-measuring unit 25 is seen more specifically inFIG. 6. This unit 25 comprises said pair of photoelectric cells, as wasreferred to hereinbefore. In practice, these cells are in the form ofphototransistors which are held in position by means of a holder 44which is formed with two light beam guide passages 46 and 47 parallel toeach other. The holder 44 is fitted with a top cover 48, a main casingand a bottom cap 51.

The top cap 48 is formed with a pair of light beam inlet openings 53 and54 in vertical registration with said passages 46 and 47, respectively,and fitted through a filter piece 52 onto the top screwed end 44a byscrewing. Between the main casing 50 and the holder 44, there is formeda cooling chamber 55 for effectively preventing any possible damage ofthe unit 25 by radiating heat energy from the plasma arc. For thispurpose, the main casing is provided with a coolant inlet 61 and anoutlet 58. Bottom cap 51 is formed with an opening 60 through whichleads 26- 29 are taken out from the phototransistors 25aand 25b.

It would be conceivable that there can be an intermediate position wherelight beams from the tail flame arrive at the inlets of bothphototransistors. However, this cannot practically been realized, thanksto the directional design of the angle measuring unit 25. For thispurpose, the provision of the light inlet openings 53, 54; the lightfilter 52 and the light beam guide passages 46, 47 having a longerlength is highly effective. If such an intermediate operation positionshould occur, no practical effect will be impressed upon the controlcircuit 35, as will be more fully described hereinafter. The diameter ofeach of thephototransistors amounts generally to about 2.5 mm.; and,theseparating distance between'them generally set to about 0.5 mm.,although in FIG. 6 they are seen as if they are brought into lateralcontact with each other. These dimensions can naturally be modifiedaccording to the thickness of the welding stock, torch travel speed,size of the plasma arc and the like various welding conditions. Sincethe diameter of the plasma amounts generally to about 2 mm., and the gapbetween the both phototransistors is selected to a highly smaller sizesuch as 0.5 mm., as above referred to, there is no risk in practice thatany light beam from the tail flame cannot arrive at the light inlets ofthe both transistors.

Next, referring to FIG. 7, the electric and electronic circuits employedin the automatic control device according to this invention will now bedescribed. I

In this figure, the photoelectric angle-measuring unit, briefly denotedas angle sensor includes said phototransistors 25a and 25b which may ofthe type: FPT (General Motor Company's Standards which may apply equallyother main circuit components). However, the sensitiveness of the firsttransistor 25a is selected stronger than the second transistor 25b,preferably in the ratio of 2:1, as an example. Two return leads 27, 28from the both phototransistors are jointed to a junction 62 from which acommon lead 63 extends through a resistor R1 to a further junction 64.

The lead 26 is electrically connected with a series of four transistorsTRI (typez2N2 l 7); TRZ (type:2N2 l 7); TR3 (type: 2N223) and TR4 (type:2N223). Transistor TRI has the same characteristic with that of thephototransistor 25a, and connected in circuit so as to balance out thedark current and for making necessary temperature compensation.

Transistors Tr2 and Tr3 are arranged for performing an amplification ofthe signal output from the first phototransistor 25a. The last stagetransistor Tr4 is arranged to serve for the :necessary temperaturecompensation of the preceding amplifier transistors Tr2 and Tr3.

The lead 29 from the second phototransistor 25b is connected equally asbefore with a series of four transistors Tr5, Tr6, TH and Tr8. Asrnay beeasily supposed the types and the arrangements of'these transistorscorrespond respectively to the aforementioned transistors Trl-Tr4, theoperational conditions of which are controlled by various resistorsR2-R5.

, In the similar way, the latter series of transistors Tr5-Tr8 aredefined in their operational mode by a plurality of resistors The outputof the final stage transistor Tr4 of the upper amplifier section isconveyed to a junction 68 through a lead 67 which contains no resistor,while the output of the final stage transistor Tr8 is conveyed to thesame junction 68 through a lead 66 which contains, however, a resistorR12 of 400 ohms selected in this case. Therefore, it will easily be seenthat the signal output from the first phototransistor 25a can beconveyed in a more easy way to the output junction terminal 68 than inthe case of the output from the second phototransistor 25b.

The voltage output from the amplifier section, so far described, of thecomparator, is conveyed through a fixed resistor R13 and an adjustableresistor R14 to a condenser CI for being charged therein and when thecharged voltage to a certain value, Vp shown in FIG. 8 and defined bythe characteristics of a unijunction transistor Tr9, the condenser isdischarged and so on, as shown at (A) in FIG. 8. The output from theunijunction transistor Tr9 is supplied to a transformer T1 and acorresponding series pulses is delivered therefrom through leads 70, 71and 72 to a rectifier bridge circuit DR2.

The oscillator arrangement as well as the function of such or pulsegenerator which includes in this case aforementioned circuit elements:R13, R14, C1, Tr9, Tr l and a further resistor R15, is well known tothose skilled in the art and thus no further analysis will be necessaryfor better understanding the nature of the invention.

It should be, however, noted that the charging time of the condenser C1depends upon the outputs from the first and the second phototransistors25a and 25b, of which the output from the former is naturally strongerthan that from latter.

For the comparator schematically shown at 30 in FIG. l'and comprisingthe amplifier and the oscillatorso far shown and described, a DC powersource is provided as shown in FIG. 7, said power source comprising atransformer T2 connected through leads 74, 75 to AC power source, 100volts, as shown. The current from the secondary winding side, 12 voltsas an example, is fed to a rectifier bridge DR3 from which an outputlead 76 extending through several junctions 73, 72, 65 and 64 to thefirst junction 62 for effective feed of current to the necessary feedingpoints in the said comparator as shown. The DC power source containsseveral conventional circuit components such as fixed resistor T16,condenser C2 and diode D1, for ripple-removing and the like purposes asconventionally.

The AC power source is connected through leads 77 and 78 to a rectifierbridge DRl which comprises, although not specifically shown, fourdiodes, A15B 4, the outlet therefrom being fed through an adjustableresistor R17 to the stator winding 45a of said drive motor 45 for theenergization thereof. Resistors R18 and R17 are provided for the initialadjustment of the field coil 45a. In the neighborhood of the latter,there are provided an adjustable resistor R19 and a coil L1 connected incircuit as shown, these circuit elements serving as adjusting means forthe control of the bias of a magnetic amplifier, generally shown at 100in FIG. 7, which will be described in a more specific way hereinafter.

The AC IOO-volt power is impressed from the AC power source throughleads 77, 78 to the primary winding :1 of a transformer T3, the inducedsecondary voltage being fed to rectifier bridge DR2 which comprises fourdiodes, A15BX2 and C11A 2, the output from this rectifier bridge beingcontrolled by the pulse series fed from the pulse generator includingtransformer T1, through lead means 69-71. Ripples contained in theoutput is suppressed by means of a condenser C3 which is connected incircuit, together with a fixed resistor R19 and diodes D2 and D3. Inthis circuit, the maximum Vmax shown in FIG. 7 is set to a certain fixedvalue, such as 80 volts.

The output from this constant maximum voltage circuit is conveyed to acontrol winding L2 which is arranged as magnetically coupled withworking windings AW 2 and AWl of a self-saturating magnetic amplifierwhich comprises further a rectifier bridge DR4 containing four diodes(pH C 22/4 Siemens). Therefore, a controlled current will be fed to acontrol winding 45b of drive motor 45. Circuit elements D4, R20 and R21are connected in circuit as shown.

Should both phototransistors 25a and 25b receive light from the tailflame, the first one 25a will act predominantly as was referred to, andthus the operation of the second phototransistor being neglected withoutprejudice.

When the first phototransistor 25a is energized, the more dense pulseseries is delivered from the transformer T1 through lead means 69-71 tothe bridge DR2. In this way, an intensified control current will be fedto control winding L2 and a correspondingly increased drive current willbe fed from the magnetic amplifier 100 to the control winding 45b,thereby the drive motor speed and thus the travel speed of torch unit 10relative to the stock 29 will be correspondingly increased.

On the other hand, when the second phototransistor 25b is energized, amore rough-pitch series of pulses is fed to the rectifier DR2 and thusthe drive current to the motor 45 will be correspondingly retarded forspeed-down of the travel speed of the torch unit 10.

It will be seen the arrangement can be modified so that the weldingstock 29 travels in place of the torch unit 10.

The control mode is modified in such a way that the feed rate of plasmagas to the torch unit 10, in place of the travel speed of the carriage12, is controlled by the output from either of the sensingphototransistors 25a and 25b.

For this purpose, a control coil L12 which is provided at the outputpart of the'amplifier section shown in FIG. 7 by imaginary lines ismounted on a control bar which is mounted pivotably at an intermediatepoint between the both ends of the bar and on the top of a conventionalresilient mount 81. The root end of the bar 80 is kept in its balancedposition, as shown, by a pair of balance springs 82 and 83. A pair ofstationary magnetic pieces 84 and 85 are mounted within the housing at85 of a gas flow control valve unit 86 which is formed with a gaschamber 87, the free end 80a of the pivotable control bar 80 extendinginto the gas chamber, so as to cooperate with either of reduced controlopenings 88 and 89 provided in the gas chamber in an opposite manner andseparated a small distance from each other. The first control opening isfluidically connected through a feed piping 90 with a supply reservoirof the plasma gas or gas mixture, while the second control opening 89 isfluidically connected through feed piping 16 to the plasma torch unit10. The bar 80 is sealed off by a pair of resilient sealing means 91 and92 from the gas chamber 87.

According to the energization of either of said both phototransistors25a and 25b, a higher or a lower control voltage is fed to the controlwinding L2, thus the plasma gas feed rate from its reservoir throughfeed piping 16 being controlled as the occasion demands.

For control of the welding current, the circuit arrangement shown inFIG. 10 is utilized.

The control pulse series is fed through lead means 69-71 to a rectifierbridge DRS which is of the similar design and arrangement shown at DR2in FIG. 7. AC current is fed from one of three phase windings L10, L11and L12 to the primary winding :11 of a transformer T4 and the inducedcurrent in the secondary winding :12 of the same transformer T4 is fedto said rectifier bridge DRS, the output therefrom being fed through anadjustable resistor R22 to a control winding L2" which is the controlmember of a magnetic amplifier the latter being of the three-phase type.windings L14-L16 are electromagnetically coupled with said three-phase(primary) windings L11-L13. The induced currents are fed throughrespective leads I, II and III to the respective component group I, IIand III, and upon rectified by passage through a pluralityof diodes -140and an arc-stabilizing unit 14l.to the torch unit 10, for control of thearc current.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In an automatic control device for plasma arc welding of a weldingstock, comprising a plasma torch movable relative electrically connectedwith the negative side of said voltage source; said welding stock beingelectrically connected with the positive side of said voltage source foracting as an anodic electrode; and a plasma gas supply means fluidicallyconnected to the interior space of said torch for the formation of a jetstream directed form said torch towards the welding stock, said jetstream being transformed into said plasma when electrical high potentialis applied from said voltage source between said cathode and said anode;the improvements comprising in combination:

1. photoelectric sensor means comprising a plurality of photoelectricelements, preferably phototransistors, arranged at a position apart adistance from the backside of said welding stock and substantially inopposition to said torch;

2. an electric control means electrically connected with the output ofsaid sensor means for receiving signal outputs from said photoelectricelements for comparison and for delivery-of the thus compared result asa measure for control of a welding parameter;

. carrier means having a drive motor for carrying said torch and beingguided to travel in the direction of said welding seam;

4. a pulse generator electrically connected with said sensor means forthe generation of a series of pulses, the pulse other hand; thereofbeing modified by the sensed output from said sensor means;

5. a magnetic amplifier electrically connected with a power source onthe one hand and with said motor on the other hand and 6. a controlwinding connected electrically with said pulse generator and coupledwith said magnetic amplifier for controlling the output from saidamplifier to said drive motor for controlling the rotational speed ofthe latter.

2. The device as claimed in claim 1, further comprising a control valveunit inserted in a plasma gas feed piping for said torch, said valveunit comprising:

1. a control winding electrically connected through amplifier means tosaid sensor means;

2. a pivotable control bar acting as a control valve member adapted forcontrol of the gaseous flow through said feed P p g;

3. a resilient mount provided within said valve unit and mounting saidcontrol bar at an intermediate point between the both ends thereof.

3. The device as claimed in claim 1, further comprising a magneticamplifier arranged between said torch and a power source for the latter;and a control winding electrically connected with saidsensor means andcoupled with said amplifier, for control of the currents fed to saidtorch, according to the sensed angle of the tail flame of the plasmaflame.

1. In an automatic control device for plasma arc welding of a weldingstock, comprising a plasma torch movable relative to the welding sockand along the welding seam; a DC voltage source; a cathodic electrodemounted within said torch and electrically connected with the negativeside of said voltage source; said welding stock being electricallyconnected with the positive side of said voltage source for acting as ananodic electrode; and a plasma gas supply means fluidically connected tothe interior space of said torch for the formation of a jet streamdirected form said torch towards the welding stock, said jet streambeing transformed into said plasma when electrical high potential isapplied from said voltage source between said cathode and said anode;the improvements comprising in combination:
 1. photoelectric sensormeans comprising a plurality of photoelectric elements, preferablyphototransistors, arranged at a position apart a distance from thebackside of said welding stock and substantially in opposition to saidtorch;
 2. an electric control means electrically connected with theoutput of said sensor means for receiving signal outputs from saidphotoelectric elements for comparison and for delivery of the thuscompared result as a measure for control of a welding parameter; 3.carrier means having a drive motor for carrying said torch and beingguided to travel in the direction of said welding seam;
 4. a pulsegenerator electrically connected with said sensor means for thegeneration of a series of pulses, the pulse pitch thereof being modifiedby the sensed output from said sensor means;
 5. a magnetic amplifierelectrically connected with a power source on the one hand and with saidmotor on the other hand and
 6. a control winding connected electricallywith said pulse generator and coupled with said magnetic amplifier forcontrolling the output from said amplifier to said drive motor forcontrolling the rotational speed of the latter.
 2. a pivotable controlbar acting as a control valve member adapted for control of the gaseousflow through said feed piping;
 2. The device as claimed in claim 1,further comprising a control valve unit inserted in a plasma gas feedpiping for said torch, said valve unit comprising:
 2. an electriccontrol means electrically connected with the output of said sensormeans for receiving signal outputs from said photoelectric elements forcomparison and for delivery of the thus compared result as a measure forcontrol of a welding parameter;
 3. carrier means having a drive motorfor carrying said torch and being guided to travel in the direction ofsaid welding seam;
 3. The device as claimed in claim 1, furthercomprising a magnetic amplifier arranged between said torch and a powersource for the latter; and a control winding electrically connected withsaid sensor means and coupled with said amplifier, for control of thecurrents fed to said torch, according to the sensed angle of the tailflame of the plasma flame.
 3. a resilient mount provided within saidvalve unit and mounting said control bar at an intermediate pointbetween the both ends thereof.
 4. a pulse generator electricallyconnected with said sensor means for the generation of a series ofpulses, the pulse pitch thereof being modified by the sensed output fromsaid sensor means;
 5. a magnetic amplifier electrically connected with apower source on the one hand and with said motor on the other hand and6. a control winding connected electrically with said pulse generatorand coupled with said magnetic amplifier for controlling the output fromsaid amplifier to said drive motor for controlling the rotational speedof the latter.